patch 8.2.1297: when a test fails it's often not easy to see where

Problem:    When a test fails it's often not easy to see what the call stack
            is.
Solution:   Add more entries from the call stack in the exception message.
diff --git a/runtime/doc/cmdline.txt b/runtime/doc/cmdline.txt
index ecaa73e..8e4e2a0 100644
--- a/runtime/doc/cmdline.txt
+++ b/runtime/doc/cmdline.txt
@@ -876,7 +876,7 @@
 			       *:<cword>* *<cword>* *:<cWORD>* *<cWORD>*
 			       *:<cexpr>* *<cexpr>* *:<cfile>* *<cfile>*
 			       *:<afile>* *<afile>* *:<abuf>* *<abuf>*
-			       *:<amatch>* *<amatch>*
+			       *:<amatch>* *<amatch>* *:<stack>* *<stack>*
 			       *:<sfile>* *<sfile>* *:<slnum>* *<slnum>*
 			       *:<sflnum>* *<sflnum>* *E499* *E500*
 Note: these are typed literally, they are not special keys!
@@ -903,12 +903,16 @@
 		   events).
 	<sfile>    When executing a ":source" command, is replaced with the
 		   file name of the sourced file.  *E498*
-		   When executing a function, is replaced with:
-		   "function {function-name}[{lnum}]"
-		   function call nesting is indicated like this:
-		   "function {function-name1}[{lnum}]..{function-name2}[{lnum}]"
+		   When executing a function, is replaced with the call stack,
+		   as with <stack> (this is for backwards compatibility, using
+		   <stack> is preferred).
 		   Note that filename-modifiers are useless when <sfile> is
-		   used inside a function.
+		   not used inside a script.
+	<stack>	   is replaced with the call stack, using
+		   "function {function-name}[{lnum}]" for a function line
+		   and "script {file-name}[{lnum}]" for a script line, and
+		   ".." in between items.  E.g.:
+		   "function {function-name1}[{lnum}]..{function-name2}[{lnum}]"
 	<slnum>	   When executing a ":source" command, is replaced with the
 		   line number.  *E842*
 		   When executing a function it's the line number relative to